White light emitting LEDs generally comprise a blue emitting LED combined with a phosphor layer that is stimulated by the blue emission of the LED into emitting yellow/red light, the combination of the yellow/red and blue emissions providing a white light. For normal direction, vertical to the surface of the LED die or vertical to the surface of the phosphor layer with an emission angle of 0°, the direct path length (not scattered) in the phosphor layer of the light rays emitted by the blue emitting LED is equal to the thickness of the phosphor layer. For increasing emission angles the direct path length for blue light rays increases. Accordingly the fraction of absorbed blue light rays by the phosphor layer is lower for the light rays with an emission angle of 0° than for the light rays with an increasing emission angle. Since the converted light emitted by the phosphor layer always has an almost Lambertian over angle distribution, the white light emitted by the LED contains more blue light emitted by the LED leading to a higher correlated color temperature for small emission angles around normal emission with an emission angle of about 0-30°. Generally, the phosphor layer is a Y3Al5O12:Ce3+ (YAG:Ce). In case of such a YAG:Ce phosphor layer emitted light becomes yellowish with increasing emission angle, perceived as yellow ring. To solve the yellow ring problem it is known to increase the scattering power of the phosphor layer and/or to add a scattering layer on top of the phosphor layer. For both, the reduction of the yellow ring problem results in a reduction of the LED efficiency, since scattering is accompanied by light reflection leading to light losses. In particular, scattering of the down-converted phosphor emission leads to reflection with accompanied reflection losses.
E.G. in the European patent application 09175903 a filter layer is introduced which comprises alternating layers of a high-refractive and a lower-refractive material.
However, there is still the continuing need for further improvement of LED assemblies to further overcome the above-mentioned drawbacks and to provide a more uniform emittance profile of the LED assembly.